This invention relates to an improved steering column assembly.
There are many applications in which it is desirable for a steering column apparatus to be telescopic so that the length of the steering column can be adjusted. By allowing part of the shaft or shroud to move telescopically over another, the steering assembly can be adjusted for reach. In a crash, where a driver impacts the steering wheel, this can also allow the wheel to move and by controlling the rate at which the telescopic movement occurs, it is possible to absorb the energy in the crash in a controlled manner.
In a typical arrangement the steering column includes a telescopic shroud having an upper and a lower tubular shroud portion. There is a slit in the top surface of the upper tube which extends along a large portion (usually more than half) of its length and which enables the upper tubular portion to be squeezed into firm contact with the lower tubular portion by means of a driver-operated clamping mechanism which is assembled around a so-called adjustment clamp bolt. The clamp bolt is typically able to slide in a generally vertical direction in slots in a vehicle mounted so-called Rake Bracket in order to make the steering wheel adjustable for height. The steering wheel actually adjusts in an arc centred around a Pivot Axis which is usually designed as part of the lower mounting means where an optional Electric Power Steering (EPS) unit mounts to the vehicle at the lower end of the overall Column and EPS assembly. The Clamp Bolt also passes through generally horizontal slots in the clamp brackets which form an upper part of the upper tube in order to allow the steering wheel to be adjusted for Reach.
Where in electric power steering unit is incorporated into the column, in a common arrangement the upper tube, by which we mean the one nearest the steering wheel, is the outer tube with the lower tube slidably located within the upper tube. It is conventional to support the upper column shaft by one ball bearing at the upper end of the upper column tube and also lower down via a splined sliding interface with the Input Shaft of the EPS unit. The EPS Input Shaft is cantilevered from two closely spaced bearings within the gearbox of an EPS unit. The resistance to bending of the pair of shafts, i.e. the Upper Column Shaft and the EPS Input Shaft critically depends on the bending stiffness of the said sliding interface which, because it must slide freely to allow the upper column to telescope, must possess some, albeit small, clearances. This method of supporting the Upper Column Shaft, while economical, makes it difficult to achieve the minimum natural vibration frequency target (typically 50 Hz) which vehicle manufacturers usual impose. Historically, this target has been aimed at minimizing steering wheel shake due to engine vibrations or road roughness. More recently, some vehicle manufacturers have been specifying still higher targets to avoid steering wheel shake during the operation of automatic engine stop-start fuel saving strategies.